If you have already done your ADSC then you should be already attending training nights, if not get hold of one of the staffs at your unit. They have introduced a system where you can attend training nights unpaid to keep your interest level up. Once there have a chat with a staff about the calender and they should be able to show you the up coming dates for the next start of phase 1 etc along with when the two week blocks run.
There is no set time period as it is run on set dates etc, so you could just miss one and wait 5 months or end up 2 weeks away from the next start. Your only real time limit is the 8 week maximum gap allowed between phase 1 and 2.
ADSC is selection. Phase 1 is the initial part of your training, also known as TSC Alpha. Once you have passed selection & been attested, which is now done at the ADSC, you will sign the paysheet at your unit when you attend training nights & you will be paid. Depending on availability of places on TSC courses & your availability to attend, it could be anything from a few weeks to a couple of months before you attend phase 1 training.
I go to Pirbright tomorrow and was called today to see if I had any questions. I asked about this and from what I am told is that UK born candidates are attested. Commonwealth candidates could be but may have to wait for security checks to be done.